I'm currently working on an oil and gas  subsea project whereby the client plans to tie-back a two-well marginal field (to a common manifold, and then) to their host facility, which is  some 8 miles away.

As part of the given operating philosophy, they'd like to have the pipeline piggable. The easiest way, in my opinion is to construct a dual-flowline loop that will allow the pig to be deployed  roundtrip.

Think of it again, I doubt that this will be the most optimized solution from economic perspective, given that this is a marginal field project. Two alternatives that come to my mind is bi-directional pigging and subsea launcher system. My concern, however, is on the operability of these options (and the inherent difficulties)

Is anyone in this forum with experience in such application, who could share their thought?

The only problem for bidirectional pigging is turning the flow around and finding the room for building the liquid, wax & crud handling tanks and drain systems etc. on each end.  If you can do that, there is no need for 2 pipelines, so you've just saved somewhere around 5 to 10 million dollars.  If you like, you can send me 10% of the savings for that little pearl of advice.

You can easily make launchers and receivers that will operate in both capacities.  Attend to the proper placements of drains and kicker lines.

You should try to eliminate any changes in line diameter between the launchers/receivers and, of course, NO CHECK VALVES between the launchers.  If you need check valves in other locations, a bypass around those will be needed.  Otherwise the usual pigging pipeline design requirements can be followed (no protrusions, orifices, other restrictions, into the line, etc.).  If you have two phase flow, pay attention to the slopes and possible liquid holdups in each direction and resulting volumes of possible slugs, ie, do a bidirectional hydraulic analysis.

There are corrosion monitoring coupons, or even electronic corrosion monitors that just take a weldolet and a wire or two to install.  I would think you could economically handle corrosion loss in this situation by giving the wall thickness a greater allowance to compensate in advance for the estimated loss.  If its a marginal field, it doesn't sound like the life will be too terribly long to worry about it chewing it up.  If it was to be a 50 yr life pipeline, that's another story.

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